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26 August 2008 Organic photovoltaic cell based on benzoporphyrin with p-i-n junction
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We have fabricated organic photovoltaic cell using tetrabenzoporhyrin (BP) as a donor material. Tetrabenzoporhyrin is formed by thermal conversion of the soluble precursor that has four bicyclo rings. Upon heat treatment above 150°C, the precursor molecule is converted to semiconductive tetrabenzoporphyrin, which is insoluble against conventional organic solvents. Taking advantage of this insoluble character of BP, p-i-n bulk heterojunction photovoltaic cell is successfully fabricated from solution, with BP/BP:fullerene/fullerene trilayer, where p-layer is crystal BP, i-layer consists of both BP and fullerene, and fullerene acts as n-layer. We have reported the p-i-n photovoltaic cell using [6,6]-phenyl-C61-butyric acid n-butyl ester (PCBNB) as an acceptor. This solution-processed p-i-n device has achieved power conversion efficiency as high as 3.4% (Jsc=9.8mA/cm2; Voc=0.62V; FF=0.56). The performance of BP device with p-i-n junction is further improved by introducing a new fullerene for the i- and n-layers. Taking into consideration LUMO level, solubility and thermal properties, a novel fullerene derivative, 1,4-bis(dimethylphenylsilylmethyl)[60]fullerene (SIMEF), is designed and synthesized. This new di-adduct fullerene , SIMEF, improves power conversion efficiency up to 4.1% (Jsc=9.1mA/cm2; Voc=0.76V; FF=0.59). This improvement is largely due to an increased Voc. We have analyzed BP:fullerene composite films to find that formation of meta-stable phase of BP is fully suppressed by SIMEF. Depth profile of this p-i-n cell is measured by means of TOF-SIMS method to observe the expected vertical distribution of the donor and acceptor that has been designed.
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Yoshiharu Sato, Takaaki Niinomi, Yoko Abe, Yutaka Matsuo, and Eiichi Nakamura "Organic photovoltaic cell based on benzoporphyrin with p-i-n junction", Proc. SPIE 7052, Organic Photovoltaics IX, 70520J (26 August 2008);

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